If you make an offer through 'buy now' that is considered a legal sale. If you email them directly and tell them you will buy it then you view it and you don't want it...basically your words mean nothing until you pay them.
Trademe has some power to enforce a sale if it is done through their site. If you do the sale off-site it is just a normal sale and you and the buyer negotiate and agree. If you say you will buy then you change your mind, not much the seller can do. But on the other hand they could also sell it before you arrive = no money no car.